It is a sleeve that holds a Glock 17 magazine and has a clip so you can place a backup magazine in your pocket for concealment.
Added a new version with better retention and a little more clearance for your fingers to remove the magazine.
Print with any filament you choose. I aligned it so the length was colinear with the Z-Axis and used support with .25mm layers. The pocket clip will be bonded to the body after printing, I broke it off by sliding an exacto knife under it.